Its quite simple... In fact I think its simpler to go to the police station than do it online.
I went to the Main Police Headquarters (CID) near Al Qusais near the metro station. I did not carry a photo. Just my Emirates ID. I went in the morning just when they opened at 8 am. I took the token and waited for 5 min. In 5 min I was called since no one else was there. Asked me why I wanted the certificate. Told him its for Canada immigration. The officer took my emirates id. Ran it through his system. He took the picture off my Emirates ID/Police system. Paid him the fee. Told me I will get in email the same day or next day. SInce I went in the morning, I guess I was lucky that the same day afternoon at about 2pm, I got the email with the digital copy. CIC accepts digital copy of Dubai PCC so thats not an issue. The whole process at the Police HQ took 15 min. Super Easy!